Reactive Oxygen Species (ROS) Are Generated as Organic Contaminants Are Broken Down: SnO2-Doped Graphene Oxide Nanocomposites Are Investigated Under Visible Light Illumination for their Photocatalytic Activity

Our aim is to establish a more efficient and reliable method for the bio-fabrication of pure SnO2 and SnO2-doped graphene oxide nanocomposites through a green chelating agent called Moringa Oleifera extract by sol-gel method. A sintered SnO2-doped GO nanocomposite exhibited increased crystallinity and size with increasing temperature, as determined by XRD studies. An FTIR investigation shows that the SnO2-doped GO nanocomposite exhibits two distinct bands at 733 cm−1 and 438 cm−1 due to terminal oxygen vibrations, while samples treated with G-O-Sn-O exhibit bands at 733 cm−1 due to antisymmetric stretching. By increasing the SnO2 peak, SnO2 nanoparticle sizes decrease, which results in a broadened GO, as well as a reduced IR intensity. By SEM and EDAX, the size and morphology of SnO2-doped GO nanocomposites were clearly observed. It was calculated that the optical bandgap of SnO2-doped GO nanostructures is 4.48 eV. It has been demonstrated that SnO2-doped GO nanocomposite can be used as an organic photocatalyst against organic pollutants methyl orange (MO) dye; despite its fast charge recombination when illuminated with visible light, these findings have been reported in detail.


Introduction
Materials with nanostructures are the subject of nanoscience and nanotechnology. They are synthesized, characterized, and exploited to improve their qualities [1]. Among them are metal oxide nanocomposite, semiconductor nanocrystals, nanotubes, nanowires, and nanobiological systems. Solar cells that are dye-sensitized are cheaper, more efficient, and can be readily manufactured when compared with conventional solar cells [2]. A photoanode, which is involved both in separating charges and generating charges, holds a significant position in a DSSC as it relates to PCE. As a result of recent industrialization, wastewater has become more and more hazardous to the environment [3]. The recent global energy crisis has also made the purification of the produced wastewater a growing concern [4]. Due to its non-toxicity, strong oxidizing power, and chemical inertness, SnO 2 is the most suitable semiconductor material for water purification [5].
The past few decades have seen a variety of approaches implemented to reduce toxic pollution [6]. An effective semiconductor photocatalyst has been suggested for the complete degradation of organic pollutants in industrial wastewater. A sustainable solution for wastewater treatment is to use photocatalysts containing semiconductor nanomaterials to degrade organic pollutants [7]. Photocatalysts made from semiconductors, however, have major disadvantages. Since TiO 2 semiconductor nanocomposite have a bandgap of approximately 3.2 eV, they only absorb ultraviolet light of the solar spectrum [8]. As a result of being exposed to sunlight, cadmium sulfide corrodes. To utilize semiconductor materials for dye degradation of organic pollutants, it is therefore a challenge to prepare inexpensive and efficient semiconductor materials [9].
It is an n-type semiconductor material with a bandgap that is photosensitive, transparent, environmentally friendly, and stable chemically [10]. Sensors [11], solar cells [12], energy storage [13], electronics [14], and photocatalysis [15,16] have been repeatedly used as research subjects for SnO 2 semiconductor material in recent years. The researchers found that SnO 2 semiconductor was better in electron capture than TiO 2 or SnO 2 when used as a photocatalyst [17]. Water splitting and hydrogen production are also other areas where this potential value has been realized [18,19]. The fast charge recombination of SnO 2 semiconductor is considered to be its major disadvantage when illuminated by UV light in practical applications, despite the fact that it has proved to be a successful photocatalyst against organic pollutants. The synthesis of nanostructures of SnO 2 has used a variety of techniques, including chemical vapor deposition [20], solvothermal synthetics [21], templates [22], sol-gel methods [23], and hydrothermal routes [24]. It has been demonstrated that hydrothermal routes can generate hierarchical SnO 2 nanostructures efficiently and at low costs [25], and they also yield high yields when modified. Due to UV light's limited absorption, SnO 2 sensors are currently limited in their practical application [26,27]. SnO 2 -doped GO photocatalysts by simple chemical precipitation are few, to the best of our knowledge. The aim of this study is to synthesize graphene oxide in the presence of SnO 2 based on the co-precipitation method. A sol-gel procedure was used to prepare graphene oxide decorated tin oxide hierarchical nanorod SnO 2 -doped GO. The preparation process included a one-step GO process followed by a one-step SnO 2 -doped GO. By X-ray diffraction (XRD) analyses, morphological analyses, and Fourier transform infrared (FTIR) spectroscopy, a detailed analysis of GO-modified SnO 2 hierarchical nanoparticles samples was presented. We observed in this study, composite materials containing graphene accepts photogenerated electrons and suppresses recombination by accepting photogenerated electrons.

Materials
It was not necessary to purify any chemicals prior to use because they belonged to the analytical grade. It was purchased from Alfa Aesar, India, that tin chloride dihydradrate (SnCl 4 .2H 2 O) and sodium hydroxide (NaOH) are used in this study. It was obtained from Alfa Aesar, India, (99.99 %) graphite powder. Preparation and washing of the aqueous solutions were carried out using deionized water.

Preparation of Moringa Leaf Extracts
Using moringa oleifera leaves (MLE), we prepared MLE by mixing the leaves with 250 mL of 30% ethanol and leaving them at room temperature for 8 h, occasionally mixing manually. Filtered extracts were re-suspended in 20% ethanol after drying with Whatman filter paper. The final concentrations of 5 and 10 g/L of MLE were achieved through re-suspension in ethanol. By using the normal heating extraction method, bioactive compounds were extracted from dried leaves. A Whatman filter paper No. 10 was used to filter the extracted extracts. In order to continue the analysis, the material was stored at room temperature in a closed container.

Preparation of Sno 2 Nanocomposite
In this study, the SnO 2 NPs were synthesized by a sol-gel method. A 200-mL beaker containing 1.6 g of (NaOH) and 100 mL of water was added 1.51 g of (SnCl 4 .2H 2 O). After 20 min of sonication with an ultrasonicator, the solution was removed from the ultrasonicator. Stirring the white colored sol for 30 min followed by drying at 80 °C for 7 h and cooling at room temperature were performed immediately after the sol was obtained. Following several washes of distilled water and filtering with Whatman paper, the white precipitates were re-washed. It was then heated at 80 °C for 6 h to obtain white sol containing SnO 2 . To determine the final consistency of the product, it was ground well into fine powder and then calcined at 600 °C for 8 h.

Preparation of SnO 2 NPs with Graphene Oxide Nanocomposite (SnO 2 /GO)
A 0.5g sample of the prepared SnO 2 NPs was dissolved in 100 mL of DD water and sonicated for 30 min with stirring. In the following step, 0.1 weight percent of graphene oxide is added, followed by 15 ml of solvent. A separate addition of moringa leaf extract was made to the solution containing SnO 2 NPs. During heating at 80 °C, the mixture was continuously stirred for 2 h. Following precipitation, deionized water was repeatedly used to wash the gray colored precipitates. During the heating process, the decorated SnO 2 -doped GO nanoparticle precipitates were kept at 90 °C for seven hours. In the next step, the material was ground well and calcined at 600 °C for 8 h until fine powder was obtained.

XRD Analysis
It is one of the most powerful methods for studying solid materials to use X-ray diffraction. In addition to being nondestructive, this method provides multiple results, including atomic-level details on crystal lattices. Phase formation and identification can also be confirmed with XRD. To determine the purity and crystalline phase composition of pure SnO 2 nanoparticles synthesized, an X-ray diffraction analysis was conducted as shown in Fig. 1a. XRD patterns of SnO 2 -doped GO nanocomposites annealed at 600 °C showed that 2θ = 10° -80°. This binary SnO 2 sample appears to be highly crystalline due to the intense and sharp diffraction peaks. Particle size is a primary reason for the expansion of the XRD peaks. It is undisputed that all the diffraction patterns in the XRD indicate SnO 2 with a tetragonal rutile structure, as the phase was in close conformity with the standard data (JCPDS Card  (110), (101), (100), (211), (220); (310), (301), (321), and (222), respectively, in the pure SnO 2 nanoparticles. An XRD pattern indicating graphene's characteristic peaks at 2θ there are 25.83° associated with (002) and 40.98° associated with (100), respectively and the formation of GO nanocomposites was associated with the new peaks at 36.6°. It should be noted that the peaks of GO are well in agreement with the tetragonal structure (JCPDS Card No. 01-0646) as shown in Fig. 1b. In the presence of binary nanostructures, all diffraction peaks shift and peak intensities decrease as a result of the interaction with Sn:G:O 2 nanostructure. The average crystallite size of synthesized binary metal oxide nanocomposite samples is calculated by Scherer's formula. Also, no other characteristic peaks are observed for impurities. D hkl =0.98λ/βcosθ; where d is the average crystallite size; λ, the X-ray wavelength (0.1546 nm); θ, Bragg's angle; and β the peak position and the FWHM (full width at half maximum) obtained by putting two Gaussian curves on the measured peaks to get the true peak positions and widths of monochromatic CuK α radiation. In SnO 2-doped GO nanostructures, the average crystallite size is 28 nm. According to the XRD pattern results, graphene has a dominant peak corresponding to (002) plane, similar to graphite's (002) plane. Since the peak of graphene (002) overlaps with the peak of SnO 2 (110) in composite SnO 2 -graphene, the peak of graphene was observed. In addition, graphene inhibited the growth of SnO 2 by making its peaks weaker, indicating that graphene could suppress its growth.

FTIR Investigation
In Fig. 2, Fourier transform infrared (FTIR) spectrum of pure SnO 2 and SnO 2 -doped GO nanocomposite were analyzed in order to identify structural information of functional groups. In the original SnO 2 nanoparticles, 4000-500 cm −1 ; 1639 cm −1 C=O stretching band vibration, 1494 cm −1 , 1011 cm −1 bands, 733 cm −1 bands, and 438 cm −1 bands were observed [28,29]. It is apparent from graphene's absorption peak at 1494 cm −1 that the vibration of C=C aromatics is responsible for the absorption peak, and that of -OH stretching at 3479 cm −1 is responsible for the absorption peak. At 1763 cm −1 , there is a very small peak that indicates C=O; 1369 cm −1 is a stretching vibration. There are peaks at 1494 cm −1 and 1011 cm −1 this is caused by C-O carboxy, and C-O hydroxyl groups vibrations associated with alkoxy groups. However, peaks are seen at 3479 cm −1 for SnO 2 -doped GO 1639 cm −1 (C=O stretching vibration), and 1642 cm −1 (OH stretching vibration). In addition to vibration, C-O vibration is 1494 cm −1 (C-C aromatics), and 1011 cm −1 (C-C aromatics). The stretching vibrations of O-Sn-O and G-Sn-O range from 733 to 438 [30].  Figure 3 shows optical absorption profiles observed over 200--900-nm wavelengths using ultraviolet diffused reflectance spectrometry (UV-DRS-Shimadzu-UV-3600 plus). An analysis of the UV-vis absorption spectra of pure SnO 2 and SnO 2 -doped GO nanocomposite was performed. There are no significant differences in the optical quality of the samples in the UV region. A total of three absorbance edges were observed in the range of 230 nm and 318 nm. Overall, the GO reduction was initially monitored by recording the UV-vis absorption spectroscopy analysis. The comparison of the pure SnO 2 and SnO 2 doped-GO nanocomposite spectrum as shown in Fig. 3a-b. Pure SnO 2 exhibited an absorption peak at 230 nm, which attributed to the π-π* transitions of the aromatic C=C bonds, and there were no other peaks identified for another transition. After SnO 2 -doped GO, the spectrum exhibited other peaks, which confirmed the reduction located at around 220-318 nm [11,15,26]. The identified absorption peak at 220 nm in SnO 2 -doped GO nanocomposite are confirmed the restoration of electronic conjugation within the graphene nanocomposite that red shifted from 220 to 318 nm due to the reduction of the GO [17,19,21]. In SnO 2 -doped GO nanocomposite, the absorption edge was at 318 nm due to the photoexcitation of electrons from the valence band (VB) to the conduction band (CB). A clear shift in maximum peak position occurs towards longer wavelengths indicating blue shift after the addition of GO. With the added GO, absorption edges are increased. In the case of treated SnO 2-doped GO nanocomposite, the E g value was found to be 4.48 eV, respectively. This is evident from the fact that pure SnO 2 NPS shows a higher E g value than graphene oxide-treated SnO 2 composites [31]. Thus, SnO 2 can be tuned with respect to its bandgap by changing its dopant composition. In SnO 2 , the bandgap energy (E g ) is determined by the wavelength value that coincides with the intersection point between the horizontal and vertical parts of the spectrum.

Morphological Analysis
A diagram of the morphology of pure SnO 2 nanocomposite can be seen in Fig. 4a and b. A scanning electron microscope image shows nearly uniformly sized SnO 2 nanocomposite. SEM images revealed agglomerated particles, the size of the particles determines the agglomeration effect. In fact, XRD measurements estimated the particle size to be 28 nm. In the nanoscale, particles agglomerate more readily, in Fig. 4c and d, SnO 2 -doped GO integration reduce the size of nanocomposite but maintain their shape. In other studies, the size of the NPs has also been reported to increase following doping  Fig. 4d-f. A study of SnO 2 lattice fringes revealed that they were 0.334 nm and 0.338 nm, which correspond to the (110) planes of the tetragonal phase [32]. XRD results confirmed the measurement of lattice fringes. In addition, SEM images revealed all grown particles had high crystallinity, which demonstrates they were incorporated into the crystalline matrix of SnO 2 -doped GO without causing lattice defects. Sn, O, and G were stoichiometrically present in EDX spectra of SnO 2 -doped GO nanocomposite as shown in Fig. 5.
As shown in the XRD results, the GO peak is a result of graphene lattices. Based on SEM mappings of elemental distributions, pure SnO 2 and SnO 2 -doped GO nanocomposite were uniformly distributed with GO [33]. The GO does not presents the layered surface structure, which was similar as previously reported [34]. After modification with SnO 2 -doped GO, the composite showed a distributed particle in irregular forms, reflecting the SnO 2 nanoparticles on the surface. The EDX analysis confirmed the Sn peak, which is associated with the SnO 2 content at 82.7% wt%.

Photocatalytic Dye Degradation Test
A degradation test was conducted on methyl orange (MO) under visible light irradiation to evaluate the photocatalytic  activity of the photocatalysts. A halogen lamp (500 W) was used as the light source that produces visible light (> 420 nm) as the background illumination. As synthesized photocatalysts were added to an aqueous solution of 100 mL of MO with constant magnetic stirring. To achieve an equilibrium of MO adsorption/desorption on photocatalyst surfaces, the solution was kept in dark adsorption with stirring for 30 min. After that, the light was switched on, and the solutions were stirred continuously [35]. An UV-vis spectrometer was used to analyses the remaining photocatalyst every 30 min after centrifuging the suspension at 6000 rpm. According to the U.S. Environmental Protection Agency, MO degradation can be calculated by measuring the decrease in intensity of 463 nm characteristic band absorption. As a result of recording the absorbance of withdrawn solutions at 463 nm, the rate of degradation of MO dyes under sunlight irradiation was calculated using the equation below [36].
where C o = initial concentration and C t is reaction concentration at time t, respectively.

Photocatalytic Activity
The characterization study above clearly showed that the prepared materials were highly crystalline, and they showed enhanced visibility absorption. Decolorization of MO dye under visible light irradiation is used to evaluate the photocatalytic ability of prepared samples. An UV-vis test was conducted to determine how well the photocatalysts performed. Absorption spectra recorded by a spectrophotometer. It took 150 min to eliminate the dye pollutant after all dye degradation experiments. Increasing irradiation time gradually decreases the peak at 463 nm. A complete photodegradation of the MO dye is evident after 150 min when the dye absorbed no longer, and the blue color of the solution disappeared. In this example, C o represents the initial concentration of catalyst, and C represents the concentration of methyl orange (MO) during the reaction at time t. An experimental description of the reaction can be found in the experiment section of the presentation [36]. Various samples were measured in order to compare the photocatalytic performance. According to Fig. 6, the absorption intensity of the aqueous solution gradually decreased with increased irradiation time [37].
The figure shows graph to compare the photocatalytic degradation performance of all the prepared photocatalyst with other samples based on their kinetic performance (C/C o ). As shown in Fig. 7a, the degradation efficiency chart for the photocatalysts was prepared. As seen in Fig. 8 based on the degradation efficiency of SnO 2 NPs and SnO 2 -doped GO samples, respectively, 86% and 93% were determined. As compared to pure SnO 2 NPs (86%) in 150 min of visible irradiation, SnO 2 -doped GO photocatalyst achieved the highest degradation efficiency (93%) out of all the synthesized photocatalysts [38,39]. Two reasons can be attributed to this phenomenon. At the interface between SnO 2 and GO, efficient heterojunction is formed by forming GO nanosheets that transfer electrons/holes effectively. The efficiency of the activity can be attributed to the efficient separation of electron hole pairs created by photosynthesis. GO nanocomposite with SnO 2 -doped surfaces may exhibit higher photocatalytic activity because they have a higher specific surface area, which results in a higher surface-active site for MO dye molecules to adhere to for more efficient catalysis [40,41]. In the final composite, the highest amount of GO can harvest the lowest light absorption and act as a shield for the active sites as a result of the highest amount of GO in the final composite. SnO 2 NPs and SnO 2 -doped GO exhibit the highest photocatalytic activity among the samples. It was investigated how the degradation reaction behaves using pseudo first order kinetics. In Fig. 7b, we illustrate a linear relationship between the irradiation time (t) and the plot's slope. A pseudo first order equation function model can explain the kinetic values found for MO dye degradation, which are ln (C o /C t ) = −kt, where k is the rate constant, C o is the initial concentration (0 min), and C t is the final concentration [42]. In all cases, pseudo-first-order kinetics was used to describe the photocatalytic degradation curves.
Considering the observed results, the pure SnO 2 NPs did not achieve the maximum degradation efficiency, whereas the SnO 2 -doped GO nanocomposite photocatalyst did as shown in Fig. 8.

Proposed Photocatalytic Mechanism
According to Fig. 9, SnO 2 has a bandgap of about 3.89 eV and conduction band potential of about 0.07 V. A SnO 2 valance band can be irradiated, causing electrons to leave their condition band [38,39]. A low photocatalytic activity to organic pollutants can be achieved without combining other materials, as electrons will transition quickly to the valance band as a result of the instability of excited states [43,44]. The photo-excited electrons will transfer from SnO 2 to graphene when SnO 2 nanocomposite are combined with reduced grapheme, preventing the recombination of electrons and holes [45]. Aside from the fact that graphene has oxygen containing functional groups, it can be used to enhance a wide range of applications because of its inorganic nanocrystal properties [46]. Graphene had greater light absorption intensity and MO adsorption than some other semiconductors, such as pure SnO 2 , while SnO 2 -doped GO had a higher work function [47,48]. Various reactive oxygen species could also be formed on SnO 2 surface when the electrons were trapped by dissolved oxygen, increasing MO degradation.

Conclusion
Through a green chelating agent called Moringa Oleifera extract, we successfully synthesized pure SnO 2 and SnO 2 -doped GO nanocomposite using sol-gel method. A variety of techniques were used for characterization of the prepared materials, including X-ray analysis, UV-vis, and FTIR spectral analysis. Through the use of XRD patterns, tetragonal structures were characterized in SnO 2 -doped GO nanocomposites. As seen in the optical absorption spectrum, the prepared product has a shorter wavelength. In the FTIR spectrum, oxygen bending vibrations are observed, which indicates functional groups are present. There is no doubt that doping has affected the functional structure of graphene structural layer, and the morphologies of the particles have affected the size of the particles. Moreover, the EDAX mapping image confirms that SnO 2 nanoparticles have been successfully incorporated with GO. Graphene oxide can be applied to a wide range of applications in the energy and environmental sectors when designed properly with pure SnO 2 nanoparticles. It remains a challenge to synthesize large-scale GO nanocomposite with SnO 2 doping on a large scale. After 150 min, the absorption peak of the dye was no longer evident due to the photocatalytic activity of nanocomposite. A study demonstrating the degradability of toxic organic dyes like MO through the use of pure SnO 2 and SnO 2 -doped GO nanocomposite has been performed using an environmentally friendly route.